In recent years, a communication system that significantly increases transmission capacity by carrying out spatial multiplex communication using a plurality of antennas, has been gathering attention. A known spatial multiplex communication includes a space division multiple access (SDMA) in which communication is performed simultaneously between a base station (BS) that includes a plurality of antennas and a plurality of mobile terminals (MTs) that includes a single antenna, and a multiple input multiple output (MIMO) in which both the BS and the MT include a plurality of antennas, and communicate by simultaneously using a plurality of spatial channels.
For example, in the following Patent Document 1, a communication device (transmission device) that performs MIMO transmission using orthogonal space (partial space) is disclosed. According to the description of the Patent Document 1, the transmission device is a transmission device of a system that communicates using a spatial division multiplex transmission scheme, and if k directivities can be formed, based on a channel response between each antenna element of the transmission device transmitted from a reception device and each antenna element of the reception device, calculates a plurality of eigenvectors from a matrix of AHA (superscript “H” is a Hermitian transformation), by using a matrix A with the size of N×M in which the channel response is an element. Then, k eigenvectors are subsequently selected from the large eigenvalue, and by setting the k directivities determined by the selected k eigenvectors as the k directivities, the separation characteristics on the separation of the multiplexed signals is improved.
In the following Patent Document 2, a method of performing spatial multiplex communication between a plurality of BSs and a plurality of MTs is disclosed. According to the description of the Patent Document 2, each BS emits information that includes a beam identification signal (BS number and beam number) using a multi-beam having a narrow beam width. The MT detects a reception level of each beam for each BS, and after forming a level table in which beam numbers equal to or more than a threshold are arranged in the descending order of the reception level, transmits thereof to the corresponding BS. Each BS calculates a degree of interference of each of its own beam from the level table received from each MT, and allocates the beam (beam number) with the least amount of interference among the beams equal to or more than the threshold of the reception level to each MT. Subsequently, the BS transmits the allocated beam number and the amount of interference (information) to the MT. Then, based on the beam number and the amount of interference received from each BS, the MT selects the beam with a small amount of interference, and communicates with the BS allocated with the selected beam. Accordingly, an appropriate base station beam can be acquired by less upstream transmission information, and a mobile communication system that can transmit downstream information at high speed is realized.    Patent document 1: Japanese Patent Application Laid-open No. 2003-258770    Patent document 2: Japanese Patent Application Laid-open No. 2002-152108